1. Field of the Invention
The present invention relates to a control system for controlling a parallel hybrid vehicle.
2. Description of the Related Art
Parallel hybrid vehicles comprise an engine as a main propulsive source of a vehicle, a generator/motor capable of operating as both an electric generator and an electric motor, and an electric energy storage device such as a battery, a capacitor, or the like for supplying electric energy to and receiving electric energy from the generator/motor. For accelerating the vehicle, the electric energy storage device supplies the generator/motor with electric energy to operate the generator/motor as an electric motor to generate an assistive mechanical output to assist in the output of the engine, i.e., vehicle propulsion forces. For decelerating the vehicle, the generator/motor is operated as an electric motor by the kinetic energy of the vehicle for regenerating electric energy, and the regenerated electric energy is supplied to charge the electric energy storage device.
The voltage occurring between the positive and negative terminals of the electric energy storage device varies due to the internal resistance of the electric energy storage device when the electric energy storage device is discharged or charged. For example, when the electric energy storage device is discharged, i.e., the electric energy storage device supplies stored electric energy to the generator/motor, with the generator/motor operating as the electric motor, the voltage across the electric energy storage device becomes lower than the voltage in a steady state, i.e., the open voltage, of the electric energy storage device due to the internal resistance thereof. When the electric energy storage device is charged with the generator/motor operating as the electric generator, the voltage across the electric energy storage device becomes higher than the voltage in the steady state, i.e., the open voltage, of the electric energy storage device due to the internal resistance thereof. If the electric energy storage device comprises an electric double-layer capacitor, for example, then since the internal resistance thereof is relatively high, the above tendency manifests itself. A reduction in the voltage across the electric energy storage device when the electric energy storage device is discharged, or an increase in the voltage across the electric energy storage device when the electric energy storage device is charged, is greater as the current flowing through the electric energy storage device is greater.
On hybrid vehicles, the assistive output generated by the generator/motor operating as the electric motor and the quantity of electric energy generated by the generator/motor operating as the electric generator are determined depending on operating states of the vehicle, such as the vehicle speed. In this case, a discharging current or a charging current of the electric energy storage device may be relatively large.
If the current (discharging current) flowing through the electric energy storage device becomes large when the electric energy storage device is discharged with the generator/motor operating as the electric motor, then the voltage across the electric energy storage device greatly drops due to the internal resistance of the electric energy storage device, tending to make it difficult for the generator/motor to operate properly as the electric motor.
If the current (charging current) flowing through the electric energy storage device becomes large when the electric energy storage device is charged with the generator/motor operating as the electric generator, then the voltage across the electric energy storage device greatly rises due to the internal resistance of the electric energy storage device, resulting in the application of an excessive voltage to the electric energy storage device, which then tends to deteriorate soon.
It has been attempted to monitor the voltage across the electric energy storage device while the hybrid vehicle is in operation. When the monitored voltage across the electric energy storage device drops below a given lower limit voltage, the supply of electric energy from the electric energy storage device to the generator/motor is stopped to stop operating the generator/motor as the electric motor. When the monitored voltage across the electric energy storage device rises above a given upper limit voltage, the charging of the electric energy storage device with the generator/motor operating as the electric generator is stopped to protect the electric energy storage device.
As described above, when the electric energy storage device is discharged, the voltage across the electric energy storage device drops due to the internal resistance thereof. Therefore, even if the electric energy storage device stores an amount of electric energy large enough to operate the generator/motor as the electric motor, the voltage across the electric energy storage device is liable to drop below the lower limit voltage for thereby stopping operating the generator/motor as the electric motor. When this situation occurs, it is impossible to effectively utilize the electric energy stored by the electric energy storage device.
Conversely, when the electric energy storage device is charged by electric energy generated by the generator/motor, the voltage across the electric energy storage device rises due to the internal resistance thereof. Therefore, when generator/motor generates electric energy upon deceleration of the vehicle, the voltage across the electric energy storage device is liable to rise above the upper limit voltage for thereby stopping operating the generator/motor as the electric generator for regenerating electric energy. When this happens, it is impossible to effectively recover the kinetic energy of the vehicle in the electric energy storage device.